Recently, methods for transmitting coded data, compressed efficiently based on inter-frame prediction, are used as a method for efficiently transmitting moving image data in many cases. Those methods encode prediction parameters and predictive residual images, obtained by predicting coded images from the temporally preceding and following frames, to reduce the information amount of moving image data that has a high time correlation. In addition, those methods efficiently compress predictive residual image data through transform coding or quantization to enable the transmission of moving image data at a narrow transmission band.
Typical examples of those methods are those using compress and encode methods such as MPEG (Moving Picture Experts Group)-1, MPEG-2, and MPEG-4. Those compress and encode methods perform inter-frame prediction for input image frames through motion compensation on a basis of a fixed-size rectangular area unit, called a macro block, and perform variable-length coding for the obtained motion vector and the signal data compressed by executing two-dimensional discrete cosine transform and quantization for the predictive residual image data.
There are many methods for distributing moving image data, obtained by the compression described above, to an IP (Internet Protocol) network that uses the packet switching method and it is expected that the IP-based distribution of moving images will also become popular on wireless transmission lines.
There are a file download method and a streaming method for distributing such moving image data. In the file download method, all of a predetermined file is distributed and then reproduced. When transmitting via a wireless transmission line, the file download method is employed in many cases to allow an action to be taken against a wireless error or loss that may be generated. On the other hand, because it takes long for the file download method to start reproducing moving images, the streaming method that can start reproduction before completely receiving moving image data is also used.
However, in the file download method, it is basically required that the download be started, not from a moving image data part requested by the user, but from the start of the file. In the streaming method, too, the reception and reproduction of moving image data from any desired position cannot be started or limited in some cases if there are continuous inter-frame coded frames.
Therefore, to decode and reproduce moving image data from any desired position, the sending side sends moving image data in which intra-frame coded data is inserted between inter-frame coded data at an appropriate time interval. A technology is also developed for use on the receiving side to divide received moving image data into multiple files and record them and to decode and reproduce the moving image data from an intra-frame coded data position included in the data read from any desired file of the files (for example, Patent Document 1).
Another known method is that, when an intra-frame coded frame is inserted at an interval of a predetermined frame period and a random access point is set, a new intra-frame coded frame is inserted and an intra-frame coded frame in the immediately preceding frame period is replaced by an inter-frame coded frame before coding (for example, Patent Document 2).    Patent Document 1: Japanese Patent Kokai Publication No. JP-A-9-18881 (FIG. 1)    Patent Document 2: Japanese Patent Kokai Publication No. JP-A-11-275583 (FIGS. 3 and 8)